376 CHORDATA. 
proximal tarsals to the tibia. The foot moves in the birds 
upon an zntertarsal joint, the movement being between the 
two rows of tarsals. 
Development (Ga//us).—The true ovum of the fowl is a large yellow 
sphere enclosed in a delicate vitelline membrane. It is usually termed 
the ‘‘yolk” of an ‘‘egg.” It is fertilised at the top of the Fallopian 
tube and passes slowly down the oviduct, developing as it goes, so that 
a laid ‘‘egg” has already developed for about eighteen hours. As it » 
passes down the oviduct albumen is added to it from glands of the 
oviduct, and this is twisted by rotation of the ovum into two cords at 
the ends of the ovum (chalaze). Further down a double egg-mem- 
brane and a shell are added and the egg is then laid. 
Segmentation is, as in the skate, meroblastic and produces a small 
blastoderm resting on the yolk. On laying, the reduction of tem- 
perature causes development to cease, and in the natural condition 
it is not resumed till the full complement of eggs has been produced 
and the hen commences to “sit.” 
Fig. 270.—THREE CONSECUTIVE STAGES OF THE BLASTODERM 
OF A CHICK IN EARLY STAGES OF INCUBATION. 
(After KoLier.) 
Area Opaca. 
Area 
Pellucida. 
Blastopore. 
The blastopore is seen in the first to be crescentic, and is gradually converted 
by differential growth into a longitudinal groove which closes 
to form the primitive groove. 
If sections of the blastoderm be made it will be found, as in the 
skate, to consist of two layers, epiblast and hypoblast, and a segmenta- 
tion cavity between them. At the future hind-end, as in the skate, is a 
thickened rim, immediately behind which a crescentic hole passes into 
a cavity, the subgerminal (or, possibly, the archenteric cavity). As 
in the skate, the epiblastic edge of the blastoderm extends gradually 
round and envelops the yolk by epiboly, but in this case the extension 
is on all sides, and hence the final closure is effected at the distal 
pole (opposite to the embryo). In the future posterior region of the 
embryo the epiblast and hypoblast remain in continuity; hence the 
epiblast does not actually extend backwards at this point, but it 
sweeps round each side, converts the crescentic groove into a longi- 
tudinal one and completes an even edge beyond it. By the third 
